Conventional commercial perlite board used for built-up roof insulation contains approximately 70% perlite by weight and 30% organic materials, such as asphalt, cellulosic fiber, and starch. Normal production on a Fourdrinier board former requires that the stock drainage characteristics be controlled within a narrow range to avoid compromising production speed on the Fourdrinier machine or drying rate in the drying oven. A relatively fast drainage rate is preferred.
The formation of a perlite board having a conventional formula on a rotary vacuum former, a cylinder machine, is difficult because solids in the aqueous slurry tend to deposit at too rapid a rate on the facing wire of the rotary vacuum filter. While some control can be exercised by use of a rotating brush roll which returns the excess portion of the stock to the system, when this recycled portion exceeds approximately 10% of the mass thickness, the stock consistency equilibrium is disrupted, and control of the process is difficult or impossible.
It has also been proposed to reduce the degree of vacuum on the cylinder to control stock deposition. However, sufficient vacuum must be maintained on the drum to prevent the deposited board stock from slipping back into the vat as it emerges from the vat.
A reduction of the solids content of the aqueous slurry in the cylindrical vacuum forming machine cannot be used effectively to control the rate of stock deposition, in view of the tendency of expanded perlite to float when it is in a dilute suspension. While the production of perlite insulating board on a Fourdrinier machine has been standardized, and produces good results, the initial capital expenditure, and the physical size of a Fourdrinier machine and its associated equipment makes it desirable to develop an improved process for the production of perlite insulating board on rotary vacuum forming equipment.